1. Field of the Invention
The present invention relates generally to a transmission/reception apparatus and method for a TDD (Time Division Duplexing) CDMA (Code Division Multiple Access) mobile communication system, and in particular, to a transmission/reception apparatus and method for transmitting frames in a transmit diversity mode to conquer a time-varying channel characteristic.
2. Description of the Related Art
In general, a TDD CDMA mobile communication system refers to a CDMA mobile communication system in which a plurality of slots constituting one frame are divided into the slots for a downlink channel and the slots for an uplink channel. Meanwhile, the CDMA mobile communication system is classified into the TDD CDMA mobile communication system and an FDD (Frequency Division Multiplexing) CDMA mobile communication system in which a transmission frequency is separated from a reception frequency.
The TDD CDMA mobile communication system is further classified into a W-TDD (Wide band TDD) CDMA mobile communication system and an NB-TDD (Narrow Band TDD) CDMA mobile communication system.
The W-TDD and NB-TDD techniques have been defined by 3GPP (3rd Generation Partnership Project), an ongoing standard for the 3rd generation mobile communication. A mobile phone employing the existing TDD digital mobile communication scheme includes a GSM (Global System for Mobile) phone and a CT-2 (Cordless Telephone-2nd generation) phone.
Commonly, performance of the mobile communication system depends on how well the system can operate in a bad mobile communication environment where the channel characteristic is subject to extreme variations. The mobile communication system has used a diversity technique in order to increase transmission/reception efficiency (or throughput) in the bad mobile communication environment. The diversity technique is divided into frequency diversity, time diversity and space diversity.
The frequency diversity is a technique for transmitting the same data with two separate frequencies, and this technique employs interleaving and channel coding. The time diversity is a technique for transmitting the same data twice at regular intervals, and this technique employs a Rake receiver. The Rake receiver, comprised of one searcher and a plurality of fingers, separately receives signals having a different arrival time due to the multiple paths. The space diversity is a technique for transmitting the same data using two antennas being spaced apart.
However, when there are almost no multiple paths or a UE (User Equipment) moves slowly, it is difficult to use the frequency diversity and the time diversity. This is because it is difficult to use the Rake receiver when there are almost no multiple paths.
In this case, therefore, the use of the space diversity is recommended, which corresponds to a transmit (transmission) diversity which can be implemented at the minimum expenses with the minimum modification of the transmission hardware of the Node B (base station transmitter) and the reception hardware of the UE (User Equipment or mobile station).
In addition, there is a time switched transmit diversity (TSTD) technique using one or more antennas. The TSTD is different in principle from the space diversity.
In the TSTD technique, the transmitter alternately uses the two spaced-apart antennas for signal transmission, so that it is possible to increase the capacity of reception channels to the maximum without modifying a structure of the receiver. The TSTD technique can be divided into a closed-loop TSTD technique and an open loop TSTD technique. A space-time transmit diversity (STTD) technique is a typical example of the open loop TSTD technique.
Meanwhile, there is a beam former scheme for improving performance of radio channel. Although it has improved performance, requires a complicated transmission or reception device such as an array antenna. Therefore, it is difficult to apply the beam former scheme to the UE. Further, the beam former scheme, even though used for the Node B, cannot be applied to a common physical channel (CPCH) which is simultaneously transmitted to a plurality of UEs.
Reference will now be made to a transmitter not using the diversity technique in the W-TDD or NB-TDD mobile communication system, and a transmitter using the closed-loop or open loop transmission diversity technique in the W-TDD mobile communication system. In addition, a description will be given of a transmitter using the TSTD technique in the FDD mobile communication system.
FIG. 7 illustrates a structure of a general transmitter not employing a diversity scheme in a Node B for a W-TDD or NB-TDD mobile communication system. Referring to FIG. 7, transmission data is encoded into a coded symbol stream at a predetermined coding rate by a channel encoder 700. The coded symbol stream is interleaved by an interleaver 702, and then, demultiplexed by a demultiplexer 704 into an I channel and a Q channel, creating a pair of complex channels. Among the complex channels, the I channel, a real channel, is spread by a first spreader 706 with an OVSF (Orthogonal Variable Spreading Factor) code, while the Q channel, an imaginary channel, is spread by a second spreader 708 with an OVSF code. The OVSF codes used in the first spreader 706 and the second spreader 708 are identical to each other. The signals on the complex channels I and Q spread with the OVSF code are scrambled with a scrambling code by first and second scramblers 710 and 712, respectively. The scrambled signals are multiplexed with a midamble sequence on a time axis by a time division multiplexer (TDM) 714. Here, output signals of the TDM 714 have a frame structure of the TDD mobile communication system. The I and Q-channel signals output from the TDM 714 are provided to a first multiplier 720 and a second multiplier 722 through a first FIR (Finite Impulse Response) filter 716 and a second FIR filter 718, respectively. The I-channel signal is multiplied by a carrier signal cos(ωct) by the first multiplier 720, and the Q-channel signal is multiplied by a carrier signal sin(ωct) by the second multiplier 722, thus outputting modulated radio frequency (RF) signals. The modulated RF signals are added by an adder 724, and then, amplified by a power amplifier (PA) 726 before being transmitted through a single antenna.
However, when the channel environment between the transmitter antenna and the UE becomes worse due to the time-varying mobile communication environment, the receiver of the UE may not decode the received signal.
FIG. 8 illustrates a structure of a common Node B transmitter using the STTD scheme, an open loop TSTD scheme, in a W-TDD mobile communication system. Meanwhile, a technical report for the NB-TDD mobile communication system just mentions possible consideration of the STTD scheme.
Referring to FIG. 8, in the transmitter using the STTD scheme, serial input data is encoded by a channel encoder 800, and then, interleaved by an interleaver 802. The interleaved coded symbols are provided to an STTD encoder 804. The STTD encoder 804 STTD-encodes the interleaved coded symbols and divides the symbols into two separate signals to be transmitted through two different antennas. The two divided signals are provided to a first demultiplexer 806 and a second demultiplexer 828, respectively. The first demultiplexer 806 and the second demultiplexer 828 each demultiplex the signals provided from the STTD encoder 804 into an I-channel signal and a Q-channel signal. The I-channel signal demultiplexed by the first demultiplexer 806 is spread with an OVSF code by a first spreader 808, and the Q-channel signal demultiplexed by the first demultiplexer 806 is spread with an OVSF code by a second spreader 809. Further, the I-channel signal demultiplexed by the second demultiplexer 828 is spread with an OVSF code by a third spreader 830 and the Q-channel signal demultiplexed by the second demultiplexer 828 is spread with an OVSF code by a fourth spreader 832. The OVSF codes used by the first to fourth spreaders 808, 809, 830 and 832 are identical to one another. The signal spread by the first spreader 808 is scrambled with a scrambling code by a first scrambler 810, and the signal spread by the second spreader 809 is scrambled with a scrambling code by a second scrambler 812. Further, the signal spread by the third spreader 830 is scrambled with a scrambling code by a third scrambler 834, and the signal spread by the fourth spreader 832 is scrambled with a scrambling code by a fourth scrambler 836. The I-channel signal and the Q-channel signal output from the first scrambler 810 and the second scrambler 812, respectively, are provided to a first TDM 814, and the I-channel signal and the Q-channel signal output from the third scrambler 834 and the fourth scrambler 836, respectively, are provided to a second TDM 838. The first TDM 814 multiplexes the signals from the first and second scramblers 810 and 812 with a first midamble sequence on a time axis according to the channels. The second TDM 838 multiplexes the signals from the third and fourth scramblers 834 and 836 with a second midamble sequence on a time axis according to the channels. The signals output from the first TDM 814 are added by an adder 824 after passing through first and second FIR filters 816 and 818 and first and second multipliers 820 and 822, and then, amplified by a power amplifier (PA) 826 and transmitted through a first antenna ANT1. The signals output from the second TDM 838 are added by an adder 848 after passing through third and fourth FIR filters 840 and 842 and third and fourth multipliers 844 and 846, and then, amplified by a power amplifier (PA) 850 and transmitted through a second antenna ANT2.
The signals transmitted through the first and second antennas ANT1 and ANT2 of the transmitter are received at the UE through two different paths, so that even if only one of the two paths maintains a proper communication environment, the UE can decode the received signal, thus increasing the performance.
It is however undesirable that the transmitter uses two power amplifiers, causing an increase in the cost of the transmitter. When applied to the UE, the STTD scheme not only causes an increase in the cost of the UE, but also impedes miniaturization of the UE. Besides, in order to receive a signal transmitted using the STTD scheme, the receiver requires an additional device for receiving the signal.
FIG. 9 illustrates a structure of a common Node B transmitter using the closed-loop TSTD scheme in a W-TDD mobile communication system.
Referring to FIG. 9, transmission data is encoded by a channel encoder 900, and then, interleaved by an interleaver 902. The interleaved coded symbols are demultiplexed by a demultiplexer 904 into an I channel and a Q channel, creating a pair of complex channels. Among the complex channels, the I channel, a real channel, is spread by a first spreader 906 with an OVSF code, while the Q channel, an imaginary channel, is spread by a second spreader 908 with an OVSF code. The OVSF codes used in the first spreader 906 and the second spreader 908 are identical to each other. The signals on the complex channels I and Q spread with the OVSF code are scrambled with a scrambling code by first and second scramblers 910 and 912, respectively. The scrambled signals are multiplexed with a midamble sequence on a time axis by a time division multiplexer (TDM) 914. The output signals of the TDM 914 are distributed to first and second multipliers 916 and 930 such that the signals should be transmitted through first and second antennas ANT1 and ANT2. The first and second multipliers 916 and 930 multiply the signals output from the TDM 914 by complex weights ω1 and ω2, respectively, provided from an uplink channel estimator 944. The complex weights ω1 and ω2 are calculated by the uplink channel estimator 944 by calculating feedback signals from the respective UEs. The complex weight ω1 is multiplied by the signal to be transmitted through the first antenna ANT1, while the complex weight ω2 is multiplied by the signal to be transmitted through the second antenna ANT2. The I-channel signal and the Q-channel signal multiplied by the complex weight ω1 by the first multiplier 916 are filtered by first and second FIR filters 918 and 920, respectively, and then, modulated with modulation signals by third and fourth multipliers 922 and 924. The modulated I-channel signal and the modulated Q-channel signal are added by an adder 926, and then, amplified by a first power amplifier 928 before being transmitted through the first antenna ANT1. Further, the I-channel signal and the Q-channel signal multiplied by the complex weight ω2 by the second multiplier 930 are filtered by third and fourth FIR filters 932 and 934, respectively, and then, modulated with modulation signals by fifth and sixth multipliers 936 and 938. The modulated I-channel signal and the modulated Q-channel signal are added by an adder 940, and then, amplified by a second power amplifier 942 before being transmitted through the second antenna ANT2.
The complex weights ω1 and ω2 indicate the channel environments between the respective UEs and the antenna ANT1, or between the respective UEs and the antenna ANT2, and are calculated using the midambles received from the UEs. Actually, since the signals transmitted through the first and second antennas are received at the UEs through the same path and the midamble used to calculate the weights, it is known that the closed-loop TSTD scheme has excellent performance.
However, the closed-loop TSTD scheme also requires two power amplifiers, like the STTD scheme shown in FIG. 8.
FIG. 10 illustrates a structure of a common Node B transmitter using the TSTD scheme in an FDD mobile communication system. The structure shown in FIG. 10 is well disclosed in Korean patent application No. 98-5526, the contents of which are hereby incorporated by reference. In the existing TSTD scheme, the transmitter alternately transmits a transmission signal using two antennas, thereby obtaining a space diversity gain. Therefore, in order to support the TSTD scheme which switches a transmission signal from one antenna to another antenna, the transmission signal should have a switching point, i.e., a guard period (GP) so as to prevent a time delay due to the switching. However, in the FDD mobile communication system, a frequency for downlink transmission is separated from a frequency for uplink transmission, so that no GP exists between frames. Therefore, in order to apply the TSTD scheme to the FDD mobile communication system, a switch should be located before digital-to-analog conversion part. As a result, two power amplifiers are required.
Referring to FIG. 10, transmission data is encoded by a channel encoder 1000, and then, interleaved by an interleaver 1002. The interleaved coded symbols are demultiplexed by a demultiplexer 1004 into an I channel and a Q channel, making a pair of complex channels. Among the complex channels, the I channel, a real channel, is spread by a first spreader 1006 and then scrambled by a first scrambler 1010, while the Q channel, an imaginary channel, is spread by a second spreader 1008 and then scrambled by a second scrambler 1012. The I-channel signal from the first scrambler 1010 and the Q-channel signal from the second scrambler 1012 are provided to a switch. The I-channel signal and the Q-channel signal are digital signals. The switch switches the I-channel signal and the Q-channel signal under the control of a switch controller 1014. The switch controller 1014 controls the switch to alternately switch the I-channel signal and the Q-channel signal to a first antenna ANT1 and a second antenna ANT2 at stated periods. When the I-channel signal and the Q-channel signal are switched to the first antenna ANT1, the I-channel signal and the Q-channel signal are converted to analog signals by first and second FIR filters 1016 and 1018, respectively, and then, modulated with corresponding modulation signals by first and second multipliers 1020 and 1022. The modulated I-channel signal and the modulated Q-channel signal are added by an adder 1024, and then, amplified by a power amplifier 1026 before being transmitted through the first antenna ANT1. Otherwise, if the I-channel signal and the Q-channel signal are switched to the second antenna ANT2, the I-channel signal and the Q-channel signal are converted to analog signals by third and fourth FIR filters 1028 and 1030, respectively, and then, modulated with corresponding modulation signals by third and fourth multipliers 1032 and 1034. The modulated I-channel signal and the modulated Q-channel signal are added by an adder 1036, and then, amplified by a power amplifier 1038 before being transmitted through the second antenna ANT2.
Accordingly, the FDD mobile communication system employing the TSTD scheme also requires a plurality of power amplifiers associated with a plurality of antennas.